One more observation : I developed the technique of pressing a shift key while returning the carriage and was typing happily with the conventional thoughts about how fine this neglected looking 60 year old machine was when I noticed there was a fault line in the center of the page: every line drops a fraction of a mm when it passes the center point.

I am more confident in my diagnosis but uncertain if its worth trying to fix it.

Hey, thank you Sky! That indeed points me in a new direction. The machine is at work so I will have to examine this Monday.

I've encountered something similar on a small Dutch made machine where the action attached to the bottom of the case through rubber bushings which had disintegrated. I also replaced them with stacked washers and was mighty proud of myself until I realized the typewriter now made a high-pitched ringing sound with every keystroke!

I will try to find some more sound-isolating solution here -- despite age, neglect and a hard platen the sound is still tolerably subdued and pleasant and the thing feels like an SG-1 with a smaller footprint.